The present invention relates to a plasma generating method and apparatus.
A plasma generating method using high frequency electric discharge is used in the fields of dry-etching apparatus for microfabrication, plasma CVD apparatus or sputtering apparatus for forming thin films, ion implantation apparatus and the like. In such a plasma generating method, it is required to generate a plasma under a high vacuum in order to miniaturize the feature sizes or to control the film quality with high precision.
The following will discuss a dry etching method for microfabrication as an example of application of the plasma generating method.
The recent progress in the field of highly dense semiconductor integrated circuits is bringing about great changes equivalent to those brought by the Industrial Revolution. The highly dense arrangement of a semiconductor integrated circuit has been achieved by miniaturization of element dimensions, improvements in devices, provision of large-area chips and the like. Element dimensions are now miniaturized to the extent of the wavelength of light. In lithography, the use of excimer laser or soft X-ray is taken into consideration. To realize micro-patterns, dry etching plays an important role as lithography does.
Dry etching is a process technology for removing unnecessary parts of a thin film or a substrate with the use of chemical or physical reactions on the surface of a gas-solid phase of radicals, ions or the like present in a plasma. As dry etching, there is most widely used a reactive ion etching (RIE), according to which a sample is exposed to a high-frequency discharge plasma of a suitable gas, so that an etching reaction is generated on the sample surface to remove unnecessary parts thereof. Generally, the necessary parts or parts not to be removed of the sample surface, are to be protected by a photo-resist pattern serving as a mask.
For miniaturization, it is required to properly arrange ions in direction. In this connection, it is important to reduce ion scattering in the plasma. To properly arrange the ions in direction, it is effective to increase the degree of vacuum in a plasma generating apparatus to increase the average free stroke of the ions. However, when the degree of vacuum in the plasma chamber is increased, this presents the problem that discharge of high frequency hardly occurs.
In view of the foregoing, there has been developed a method of applying a magnetic field to a plasma chamber to facilitate discharge, e.g., a magnetron reactive ion etching technology, an electron cyclotron resonance etching technology (ECR), or the like.
FIG. 16 is a schematic diagram of a reactive ion etching apparatus using conventional magnetron discharge. Reactive gas is introduced into a metallic chamber 81 through a gas controller 82. The pressure in the chamber 81 is controlled to a suitable value by an exhaust system 83. An anode 84 is disposed at an upper part of the chamber 81, and a sample stage 85 serving as a cathode is disposed at a lower part of the chamber 81. An RF power supply 87 is connected to the sample stage 85 through an impedance matching circuit 86, so that high frequency discharge takes place between the sample stage 85 and the anode 84.
Disposed at the lateral sides of the chamber 81 are two pairs of AC electromagnets 88 of which phases are shifted by 90.degree., the AC electromagnets 88 of each pair being opposite to each other. By the two pairs of AC electromagnets 88, a rotational magnetic field is applied into the chamber 81 to facilitate discharge under a high vacuum. With such an arrangement, the rotational magnetic field causes electrons to present cycloid motions. This lengthens the motional passages of the electrons to increase the efficiency of ionization.
According to the magnetron technology or ECR technology above-mentioned, however, the plasma is non-uniform in density. This not only makes fine etching difficult, but also induces damages to a sample or workpiece.
In a conventional magnetron reactive ion etching apparatus, the rotational magnetic field averages the local ununiformity of a plasma with the passage of time, causing the plasma ununiformity to be equalized. However, since the momentary densities of plasma are not uniform, potentials locally differ from one another. Accordingly, when the conventional magnetron reactive ion etching apparatus is applied to a MOSLSI process, there is a possibility of a gate oxide layer being broken.
In an ECR etching apparatus, too, since the magnetic field is distributed in the radial direction of the chamber, the plasma densities are locally coarse and dense. This causes the etching source to be ununiform or produces local differences in potential. Due to the ununiformity of the plasma, the uniformity of etching is deteriorated, thus making it difficult to produce LSIs with high yield. When the plasma is not uniform, it means that accurate etching cannot be made when dry-etching large-diameter wafers or hyperfine pattern LSIs in which thinner gate oxide layers are used.
Also, there has been proposed a method by which high frequency electric power of 100 to 200 MHz is superposed on a conventional magnetron etching apparatus of the parallel flat-plate type to be excited with 13.56 MHz, causing the plasma to be increased in density so that the self-bias voltage is decreased to reduce damages to a sample by high-energy ions.
According to this method, the plasma density can be increased, but it is difficult to improve the uniformity of the plasma. Thus, the problems due to non-uniformity of the plasma in the above-mentioned method cannot be fully solved.
In view of the foregoing, the present invention is proposed with the object of generating a highly dense plasma excellent in uniformity under a high vacuum.